Gravimetric methods of analysis are based on the
measurement of mass. The two gravimetric methods are precipitation methods and
volatilization methods. In precipitation methods the analyte is converted to an
insoluble product, filtered, washed and heated. The mass of the resulting residue is
determined. In volatilization methods the analyte is heated and the analyte or its
decomposition product is collected. The resulting loss of mass is determined.

General form for calculations

The calculations for gravimetric analyses are
fairly straight-forward.

Gravimetric calculations are based on the fundamental stoichiometric
calculations. (Note: You may wish to review these calculations before continuing.)
The basic form of the calculation is:

The gravimetric factor (GF) comes from a
combination of the mole ratios and the formula weights used in the stoichiometric
calculation.

For example, if you were looking for SO3
and your precipitate was BaSO4, the gravimetric factor would be:

The numbers, 80.064 and 233.391, are the formula weights of SO3 andBaSO4, respectively.

The main question is how to determine the mole ratio without knowing the entire
reaction. This is actually quite easy. Simply balance the common element. Most
of the time oxygen is not considered. In the above example, sulfur appears in
both terms.

There is only one sulfur in each term and the sulfurs are balanced. In
other words, the mole ratio is 1.

Consider the following GF:

The common element is silver, Ag.

However, there are two silver atoms represented in the upper term and only one
in the lower term.

To "balance" the silver atoms, a 2 is placed in front of the
substance in the lower term.

The calculation set-up for this gravimetric factor
would be:

Ideal Properties of Precipitating
Reagents

Specific

Selective

Readily Filtered

Low Solubility

Unreactive with the atmosphere

Known composition after being heated

Size of precipitate particles

Precipitates can be classified according to the size of the particles. Colloidal suspensions are
composed of particles ranging from 10-7 cm to 10-4 cm diameter.
Particles in crystalline suspensions
are greater than 0.1 mm diameter.

Large particles are desired since they are filtered much easier. In order to
obtain these large precipitate particles we must understand the factors that affect the
particle size and use them to our benefit.

Factors Affecting Particle Size

The size of the precipitate particle is affected by four factors.

Precipitate solubility

Temperature

Reactant concentrations

Rate reactants are mixed

Characteristics of Precipitation
Reactions

Precipitation reactions are generally slow and supersaturation is very likely. When supersaturation is HIGH,a large number of small
particles is formed. When supersaturation is LOW,
a small number of large particles is formed. This results in a crystalline suspension.

Therefore, the goal in precipitation reactions is to minimize supersaturation
conditions.

Variables That Minimize Supersaturation

Elevated temperatures

Use dilute solutions

Slow addition of precipitating agent with good
stirring

Improving Filterability

The filterability of precipitates may be improved by encouraging the growth of
large precipitate particles. "Digestion" refers to this process and generally
means to allow the precipitate to form in hot, unstirred solutions.

Sometimes a precipitation reaction will only form very small particles and
result in a colloidal suspension. Quantitatively filtering colloidal suspensions is
difficult because of the small particle diameter. However, a colloidal suspension may be
converted into a filterable solid. The process of converting a colloidal suspension to a
filterable solid is called coagulation. The reverse process is called peptization.

Coagulation can be encouraged by either precipitating from hot, stirred
solutions containing an electrolyte or by letting the precipitate stand for an hour or
more in its mother liquor. (Digestion)
The mother liquor is the hot solution
from which the precipitate is formed.

Co-precipitation

Sometimes soluble compounds are removed from solution during the formation of
precipitates. This is called co-precipitation. There are four types of co-precipitation.

Surface adsorption (equilibrium process)

Mixed crystal formation (equilibrium process)

Occlusion (mechanical process)

Entrapment (mechanical process)

Drying Precipitates

After the precipitate is filtered, it is heated until its mass becomes
constant. The primary purpose of this is to drive off any solvent and any other volatile
species that may be present. The temperature and time required to produce a suitable
product varies from precipitate to precipitate.

Some precipitates are also ignited. This process decomposes the solid, whose
exact composition may not be known, and forms a new compound of known composition. This
new compound is called the weighing form.

Critique of Gravimetric Methods

Time

Gravimetric methods are slow. However, they are more efficient since they
require less operator time and no calibration. The results are calculated directly
from the experimental data and requires only the gravimetric factor and molar masses

Equipment

Equipment used for gravimetric analyses are simple, reliable, relatively
inexpensive and easy to maintain.

Sensitivity and Accuracy

The sensitivity and accuracy of many analytical methods are limited by the
devices used for measurements. For gravimetric analyses the analytical balance is the
limiting measurement device. However, other factors must also be considered, such as
solubility losses, coprecipitation errors, and mechanical losses of the precipitate.

Gravimetric methods are the method of choice, particularly if the analyte
concentration in the sample is larger that 1%. Other analytical methods should be
considered for analyte concentrations less than 0.1%.

Specificity

Reagents used for gravimetric methods tend to form precipitates with groups of
ions. In this sense they are selective. Usually they do not form precipitates with a
specific ion. This means that ions within a group interferes with the determination of any
other ion in the group unless a preliminary separation is performed.